This invention is a further development in the fabrication of dispenser cathodes, which find application generally in microwave tubes and linear beam devices.
Heretofore, the emitting surfaces of dispenser cathodes have been made either from porous metal matrices whose pores were filled with electron emitting material, or from porous metal plugs covering reservoirs of electron emitting material.
The porous metal bodies of prior art dispenser cathodes, whether they were matrices filled with electron emitting material or porous plugs covering reservoirs of electron emitting material, did not have consistently uniform pore size, pore length, or spacing between pores on the surface. As a consequence, dispenser cathodes of the prior art tended to exhibit non-uniform electron emission from their surfaces.
U.S. Pat. No. 4,101,800 (issued July 18, 1978) described a dispenser cathode comprising a reservoir of electron emitting material covered by a perforated metal foil. The pattern of perforations was such as to permit migration of electron-emitting material from the reservoir to the foil surface in such a way as to coat the surface uniformly, thereby providing a cathode surface of substantially uniform emissivity. The prior art has not, however, developed a practicable method for fabricating a perforated metal foil of the kind disclosed in U.S. Pat. No. 4,101,800. Consequently, the production of dispenser cathodes having uniform surface porosity has not heretofore been commercially feasible.
A further problem with such prior techniques has arisen from the conventional use of a barium carbonate starting material during fabrication, and its subsequent conversion into the barium oxide electron emitting material during cathode activation by heating. The carbonate thereupon converts to the oxide, giving off carbon dioxide gas. This results in the carborization or oxidation of the electron emitting metal surfaces; further, such carborized or oxidized surfaces then reduce the active barium oxide into elemental barium, which evaporates at tube operating temperatures.